It is known that natural gas may contain traces of the radioactive noble gas radon. Radon occurs in the series of products that are formed successively during the natural radioactive decay of uranium. Uranium occurs widely on earth and therefore it may be expected that radon will occur fairly generally in natural gas wherever in the world it is found. The quanity of radioactive material that reaches the earth's surface together with natural gas, however, is only 0.001% of the quantity which the earth produces elsewhere and in widely scattered places.
Radon, being a gas, will easily be entrained by the natural gas. Radon, however, is also subject to decay, yielding particles which are no longer chemically inert and less volatile than radon. It is conceivable that thus radioactive metallo-organic compounds are formed which again are so volatile as to be at least partly entrained with the natural gas. Also, during transport, decay products and compounds thereof will be deposited on walls. Obviously this occurs during the residence and the transport of the natural gas in the earth's crust. Besides, such radioactive products which emerge from a well together with the gas may tend to be deposited on parts of above-ground installations, such as pipelines, drying equipment, separators, valves, etc., which creates a potential danger of an accumulation or radioactive material in places accessible to man. Thus, it has been found that the condensate of light hydrocarbons higher than methane, recovered from the natural gas in a certain field, has a concentration of the radioactive isotope polonium 210 that is measurable through its radiation. This isotope is an alpha radiator with a half life of 138 days. This concentration, however, is very low. The activity concentration is of the order of magnitude of 2 pCi/gram (pivo Curie), which corresponds with a concentration of Po 210 of 10.sup.-15 gat/l. Chemical analysis is impossible because of this very low concentration, and the radiation level is far below the dose that is dangerous to man. Nevertheless, it is highly desirable to reduce the level of radioactivity in order to preclude any hazards due to accumulation on parts of above-ground installations, and the invention provides a process for this purpose.
The invention therefore relates to a process for reducing the level of radioactivity of a stream of light hydrocarbons by passing that stream through a space containing solid particles for the removal of radioactive metals or compounds thereof, which solid particles contain one or more metals from Group VI B, alone or combined with one or more metals from Group VIII B of the Periodic Table, in the sulfidic form, the stream of hydrocarbons being passed through at a temperature lower than 100.degree. C. at a space velocity of 0.1 to 100 kg per hour per liter of space filled with solid particles.
This process can be carried out both with gaseous and with liquid hydrocarbons. The pressure may be chosen freely. In the case of a liquid stream, such as a condensate of light hydrocarbons, a pressure of 1 bar is very suitable because of the relatively cheap apparatus that is required for it.
The surprising feature of the process according to the invention is that when the said sulfided metal-containing particles are used the temperature may be kept below 100.degree. C., while the space velovity may be varied between wide limits. It is not possible to find out exactly what happens during this process, because the initial concentration of the particles to be removed is so low. Even the quantities accumulated on the contact material are still too small for chemical analysis. The measurement of radiation is virtually the only source of information about the particles. It has been found, for example, that the level of radioactivity of condensate originating from a source of natural gas can be reduced 100 to 150 times by a process according to the invention. The possibility that has now been found of working at the very low temperature of less than 100.degree. C., in particular at ambient temperature, is very important in practice, because facilities for maintaining the desired temperature can be omitted. This is especially important with a view to applying the process away from a normal plant, such as on production fields, on artificial islands or in coastal areas where pipelines come ashore.
Sulfidation of the metals may be effected by exposure to a stream of pure H.sub.2 S at ambient temperature and 1 bar for 72 hours, followed by stripping with an inert gas such as nitrogen. Sulfidation may also be carried out with an H.sub.2 S/H.sub.2 mixture, for instance in a molar ratio of 1/7. The temperature may then be increased to 375.degree. C. in 3 hours' time and be maintained at this temperature for another hour. The pressure may be 10 bar. Cooling is effected under H.sub.2 at 10 bar. If the stream of hydrocarbons contains any sulfur compounds, sulfidation can be done in situ. If mercaptans are present, it can take place at ambient temperature.
The metals are preferably chosen from the group Ni, Co, Mo and W.
Solid particles containing sulfided metals chosen from the above-mentioned groups are well-known catalysts for various conversions or treatments of hydrocarbons such as hydrogenation and desulfurization of oil fractions. The said metals are then, as a rule, supported on carrier material, mostly inorganic oxides such as Al.sub.2 O.sub.3 or MgO. For a high catalytic activity the material should have a large internal surface area, for instance, of 150-350 m.sup.2 /g for catalysts with Al.sub.2 O.sub.3 as the carrier which means that the material has a microporous structure (average pore diameter smaller than 20 nm). The process according to the invention, when carried out with solids having such structures, has shown to give good results at ambient temperature.
Even better results are obtained when the metals chosen are supported on macroporous carrier material. Better results are also obtained when the carrier material is apolar. Both measures have a similar effect. For, it has been found that solids having a microporous structure will show a decline in activity relatively soon, a tendency which materials that are macroporous and/or apolar show only after very prolonged operation. Although this is not certain, the explanation may be that in a microporous and/or polar solid the active sulfided metals are shielded by water and/or by polar organic compounds usually present in the stream of light hydrocarbons, possibly in traces. The polar inorganic and organic materials may accumulate in the pores by adsorption and/or capillary condensation. Apolar carriers absorb only little or not at all. Materials with a macrostructure have very wide pores that cannot, or hardly, be blocked by adsorbed polar molecules. Materials with a miacroporous structure have a far lower internal surface area than of those with a microporous structure. A boundary line between the two structures is, of course, an arbitrary one, but an indication may be that the internal surface area of macroporous materials is smaller than 100 m.sup.2 /g and the average pore diameter larger than 30 nm. Experimental resuls show that the process according to the invention does not call for a large internal surface area, which is plausible in view of the very low initial concentration of about 10.sup.-15 gat of radioactive material. Suitable carrier materials are macroporous activated carbon, macroporous silica gel or diatomaceous earth.
An important, advantageous possibility is the use of solid particles consisting entirely of the metals chosen in the sulfidic form, not supported on carrier material. A suitable composition is, for instance, one with Ni, W and S in atomic ratios within the ranges of 0.01-3Ni:1W:1-4S, for instance 0.5Ni:2S.
This material without a carrier usually has a small internal surface area, less than 100 m.sup.2 /g, and it is macroporous. The material has a large capacity for reducing the level of radioactivity and does not show any sign of a decline after about 1 months' operation.
Solid particles, in particular those of a microporous and/or polar character, can be regenerated by drying. This can be done by passing a gas, e.g., N.sub.2, over the particles at a temperature of 40.degree.-150.degree. C.
A suitable means of counteracting a decline of the capacity for reducing the level of radioactivity, or at least of delaying it, is previous drying of the stream of light hydrocarbons. To this end any conventional drying agent may be used, such as CaCl.sub.2, microporous silica gel, etc. The importance of the two above-mentioned measure -- regeneration and drying of the incoming stream -- decreases according as the solid particles are less prone to a decline in absorption capacity due to the aforementioned causes. As stated before, no distinct boundary line can be drawn. For instance, the water content of the stream of hydrocarbons will also play a part.
The process according to the invention may be carried out under hydrogen pressure. However, the advantages obtained are marginal.